Hi Ponnusamy,
1. Archive and Unarchive is the data migration procedure available between any servers. If u are really concerned about doing this for the first time, you can try doing it between Sandbox and Dev servers if you have them in the landscape. If not, try doing it between two different, unimportant repositories in lower level envronments.
2. As Shamir rightly said, u can refer to the Console reference guide for Archive procedure. The number of repositories can be depending upon your DB size. However the less the number of repositories more will be the performance. Also the number of records in the repositories and the number of tables in the repository plays a vital role in the performance.
Post us your further queries:)
Kind Regards,
Thamizharasi N